This invention relates to an economical process for the production of polymerisable cyclopentene by splitting dicyclpentadiene under the effect of heat to form cyclopentadiene, partially hydrogenating the cyclopentadiene formed to form crude cyclopentene and subsequently purifying the crude cyclopentene by distillation.
Cyclopentene is a monomeric hydrocarbon which is becoming increasingly more valuable for the production of polymers, for example for the production of high quality synthetic rubber (Hydrocarbon Processing, December, 1972, pages 72 to 75).
A preferred starting material for the production of cyclopentene by way of the intermediate stage cyclopentadiene is dicyclopentadiene. The dicyclopentadiene is preferably used in the form in which it accumulates as a reaction product in petrochemical hydrocarbon pyrolysis processes. In these pyrolysis processes, the dicyclopentadiene initially accumulates in the monomeric form as cyclopentadiene. For example, the C.sub.5 -fractions of naphtha crackers contain approximately 20% by weight of cyclopentadiene. Since a considerable outlay is involved in separating off the monomeric cyclopentadiene from such fractions and since the cyclopentadiene is not stable in storage on account of its ready conversion into the dimeric form, it is converted into the dimeric form by heat treatment and subsequently isolated by distillation from suitable fractions in the form of dicyclopentadiene. Accordingly, dicyclopentadiene may be regarded as the storage and transport form for cyclopentadiene. In this form, it also constitutes for example the starting material for the production of cyclopentene.
The cyclopentadiene is subsequently recovered from the storable and transportable dimeric form by splitting under heat. Accordingly, splitting of the dicyclopentadiene into cyclopentadiene forms the first stage of the process according to the invention. There are already a number of processes for splitting dicyclopentadiene into cyclopentadiene either in the gas phase or in the liquid phase. Since all these processes are carried out at elevated temperature, one of the more significant secondary reactions accompanying splitting is resinification of the dicyclopentadiene which can actually result in coking of the reaction vessels. Accordingly, processes for splitting dicyclopentadiene under heat into cyclopentadiene always includes measures for reducing or preventing resinification or coking in order to improve the yield of the splitting reaction.
In the gas-phase splitting of dicyclopentadiene, the consumption of energy and the danger of coking of the reaction tubes are particularly serious on account of the high reaction temperatures of around 400.degree. to 500.degree. C. In order to prevent coking, some processes are carried out with considerable quantities of inert diluting gases, such as steam, nitrogen, hydrogen and gaseous hydrocarbons (DOS No. 2,127,625). It is also known that a high rate of flow and minimal residence times may be maintained with a view to obtaining high yields (DOS No. 2,102,262).
The liquid-phase splitting of dicyclopentadiene is carried out at temperatures in the range of from about 160.degree. to 240.degree. C. Accordingly, far less outlay is involved. On the other hand, splitting in the liquid phase involves longer residence times, for example of a few hours, which necessitate special precautions for reducing or completely preventing resinification of the apparatus used. Accordingly, the liquid-phase splitting of the dicyclopentadiene is generally carried out in the presence of high-boiling, inert diluents which are stable under the splitting conditions, in order to keep the resin products formed in solution. Thus, according to DAS No. 1,032,250 and U.S. Pat. No. 2,887,517, higher paraffin hydrocarbons are used as diluents. It is also known that splitting can be carried out with very low concentrations of cyclopentadiene (U.S. Pat. Nos. 2,387,993; 2,636,054 and 3,016,410).
One proven process for the liquid-phase splitting of dicyclopentadiene is described in DOS No. 2,019,596, the so-called codimers present in some streams of dicyclopentadiene (codimers are mixed dimers of cyclopentadiene, isoprene and piperylene which are very stable during splitting) being used as solvents for the resins formed. The advantage of this process is that the solvent used for the resins formed is already present in the system.
Accordingly, it can be said, in regard to the splitting of dicylopentadiene under heat to form cyclopentadiene as the first stage in the production of cyclopentene, that the most significant problem affecting this stage lies in the formation of secondary products as resins which can result in coking of the reaction vessels, and that, in addition to the actual splitting reaction, a number of measures have been proposed specifically with a view to eliminating resin formation.
The subsequent process stages of partial hydrogenation of the cyclopentadiene into cyclopentene and the further purification of the cyclopentene to the degree of purity required for polymerisation, present fewer difficulties by comparison with splitting. The yield of the dicyclopentadiene splitting stage also makes a greater contribution to the overall yield of the process for the production of cyclopentene than the following stages of partial hydrogenation and purification.
A complete process for the production of polymerisable cyclopentene is described in Hydrocarbon Processing, December `1972, pages 71 to 75. In that process, dicyclopentadiene is split in the liquid phase. The cyclopentadiene obtained is selectively hydrogenated, after which the crude cyclopentene obtained is purified in a fine fractionation stage. Dicyclopentadiene input streams with higher contents of so-called codimers (mixed dimers of cyclopentadiene, isoprene and piperylene) which, in liquid-phase splitting, serve as solvents for the resins formed, are used for the splitting reaction. To an increasing extent, however, the dicycloptenadiene fractions which have become commercially available in the meantime no longer have the high codimer content required for carrying out this process.